Resin impregnated material webs are needed for the fabrication of laminates or laminated materials, such as decorative laminates, for the surface coating of chip boards, copper-lined laminates as base material for printed circuit boards, and the like. In the fabrication of laminates, the resin impregnated material webs are arranged in layers in the desired sequence and pressed together. Normally, fiber or woven webs are used as material webs, e.g. paper webs, woven glass webs, fiber glass mats, etc. Duroplastic resins such as melamine, urea, phenol, epoxy and thermoplastic resins are used for impregnating the material webs.
Resin impregnated material webs are fabricated by winding off the material web from a roll, guiding the web through an impregnating bath and a subsequent drying channel, and winding the web on a roll which is stored for further processing. The resin which can be provided with a hardener and an accelerator, if required, is in a fluid state in the impregnating bath. The resin is dissolved in a solvent for this purpose. Water, alcohol, ester or hydrocarbons or mixtures thereof are used as solvents. If a duroplastic resin is used, this is pre-cured, to a certain extent, i.e. to the so-called A-state. The material web is saturated with resin solution when passing through the impregnating bath. The excess resin is squeezed out when it leaves the impregnating bath.
The solvent is removed from the material web in the drying channel by supplying heat by infrared radiators or a flow of hot air. The duroplastic resin is simultaneously cured to the so-called B-state. The B-state involves a reaction of the resin resulting in a stable, but still meltable state of the resin. The fabrication of the resin impregnated material web, also called prepreg, is then concluded.
During the subsequent pressing of the resin impregnated material webs which can be carried out, e.g., continuously in a double band press or discontinuously in a platen press, the duroplastic resin is finally completely cured until reaching the C-state and connects the individual layers to form the compact laminate.
A disadvantage in this method is that a solvent is required to achieve the fluid state of the resin in the impregnating bath which enables penetration into the material web. This solvent must then be removed in the drying channel, which leads to considerable environmental loading. The solvent can usually not be removed completely so that inclusions of the solvent remains in the laminate during the subsequent pressing of the material webs to form the laminate, which in turn lowers the quality of the laminate. Further, it has been shown that the application of resin is very often not uniform over the surface area, which ultimately leads to fluctuations in the characteristics of the laminate. The penetration of the resin into the fibers of the material webs is sometimes unsatisfactory, resulting in a poor bond between the individual layers of the laminate. Moreover, very long drying channels are required before the resin has cured to the B-state, which greatly increases the cost of the installations for the fabrication of resin impregnated material webs.
DE-OS 38 34 993 discloses a process in which the material web which has not been impregnated with resin is wound off a roll and placed on the portion of a press band returning to the inlet zone of a double band press from the outlet zone. Prior or subsequent to this, the material web is provided with a low-solvent resin. The returning band portion is heated so that the material web provided with resin is acted upon by heat and the impregnating and drying processes for the material web takes place on the returning band portion. The resin impregnated material web is combined with the rest of the material webs at the inlet to the double band press and then pressed together to form a laminate in the double band press under the influence of heat and pressure.
In this process, the impregnation of the material webs with resin and the pressing of the material webs to form the laminate are integrated in a one-step process. To obtain a laminate which is completely cured to the C-state in pressing, the press bands of the double band press must be operated at a feed rate which is dependent on the state and type of the resin. The impregnation and drying processes carried out on the returning band portion take place at the same speed depending on the system. This speed however is generally not the same as that at which resin impregnated material webs which are optimally cured to the B-state before entering the double band press are obtained. The pre-curing and curing conditions at the returning band portion or in the actual reaction zone of the double band press are in a ratio which is fixed by the given dimensions of the apparatus, but unfavorable for optimal curing conditions because they cannot be varied. There is accordingly a risk that laminated webs will be produced which do not satisfy demands with respect to quality.
In many cases, laminates are composed of more than two resin impregnated material webs. For example, the laminated material core of copper-lined laminates may require between eight and ten woven glass webs impregnated with epoxy resin. The impregnating and drying processes of a plurality of material webs must accordingly be carried out simultaneously on each of the two returning band portions of the two press bands of the double band press. Since the material webs are arranged, one on top of the other, it has been shown that the material webs are not impregnated and pre-cured uniformly and sufficiently. This results in low quality laminates.
Although the one-step process according to DE-OS 38 34 993 makes it possible to use low-solvent resins, this process has not been successful. Part of the reason for this is that increasingly greater quantities of prepreg which must be pressed in sheets are needed for the operation of existing production plants outfitted with platen presses, and particularly for the gluing of multiple-layer printed circuit boards. These quantities will increase even more in the future.
It is therefore an object of the invention to improve the fabrication of resin impregnated material webs so as to ensure a resin impregnation into the fibers of the material webs completely and uniformly over the surface with the use of solvent-free resin.
Another object of the invention is to provide a process and apparatus for the continuous fabrication of resin impregnated material webs.